1. Field of the Invention
This invention relates to a method for nitriding the surface of a shaped article of titanium such as a dental cast and to an apparatus for the nitriding treatment.
2. Description of the Prior Art
In recent years, use of products by the casting of pure titanium or titanium alloys is disseminating in the field of dentistry. In consequence of this trend, the practice of nitriding the surface of such a shaped article of titanium as described above thereby forming a nitride layer on the surface to increase the surface hardness and improve the abrasion resistance of the surface and, at the same time, conferring a golden color upon the surface to give an added value to the shaped article is gaining growing acceptance. Further, this nitriding treatment for shaped articles of titanium is used on industrial parts in general similarly for the purpose of increasing surface hardness and improving abrasion resistance. As means for providing nitrided surfaces for shaped articles of titanium, there have been known the gas nitriding method and the ion nitriding method. For example, Japanese Patent Publication SHO 56(1981)-44,148 discloses a method for nitriding the surface of a shaped article of titanium or an .alpha.-titanium alloy. This surface nitriding method comprises keeping a shaped article of pure titanium or an .alpha.-titanium alloy in contact with a titanium nitride powder in a chamber and feeding nitrogen gas into the chamber and heating the contents of the chamber at a temperature approximately in the range of 790.degree. to 880.degree. C. thereby causing the titanium nitride powder enveloping the shaped article of titanium to nitride the surface of the shaped article. This method, however, has a disadvantage that the nitriding treatment takes a long time because the formation of a nitride on the surface of the shaped article of titanium relies on the mutual reaction between the titanium nitride and the shaped article of pure titanium or an .alpha.-titanium alloy. As another means, the gas nitriding method has been known to the art which comprises bringing nitrogen gas into contact with a shaped article of titanium as kept in a heated state. This method effects the surface nitriding treatment by the action of nitrogen gas of high purity. Despite its high purity, the nitrogen gas inevitably contains impurities such as oxygen and nitrogen even in minute amounts. When the surface of a shaped article of pure titanium or a titanium alloy is exposed to the nitrogen gas, therefore, these impurities react with the surface to induce such adverse conversions as oxidation and absorption of hydrogen and tend to impede uniform deposition of a nitride layer on the surface of the shaped article of titanium, impair the surface strength, or cause the surface of the shaped article of titanium to assume a color different from the color desired. Removal of such extraneous oxygen and hydrogen components from the nitrogen gas necessitates use of an apparatus which is voluminous and very costly. The apparatus, therefore, constitutes a cause for obstruction in the general sense of cost reduction and enhanced adaptability.